Rubber compounds are typically made out of raw latex which is mixed with a chemical vulcanisation agent such as sulphur, peroxides, metallic oxides etc. The mixture is then heated to activate the vulcanisation process to produce a firm, elastic and durable rubber compound.
Rubber compounds are usually mixed with other chemical compounds to enhance the physical properties of the compound depending on its desired end use. In normal use, rubber compounds provide high resistance between rubber and the contact surface and also good electrical insulation. However, such properties may pose certain problems during use where electrostatic charge build up during operation can prove hazardous.
In order to increase the compound's conductivity to prevent electrostatic build-up, an electrically conductive component is added to the compound prior to vulcanisation. Examples of electrically conductive components include metallic salts, small metal parts, carbon black and sulfonic acid doped polyaniline. An increased conductivity of the compound provides an electrical pathway for electrostatic charges to dissipate to the ground.
Metallic salts and metal parts are known to affect the elasticity and durability of the non-conductive rubber.
The use of carbon black as an electrically conductive component in rubber compound is known to cause staining. This is not ideal where the compound is intended for use as a consumer product.
Sulfonic acid doped polyaniline does not cause any staining unlike carbon black but it is known to cause interference with the crosslinking efficiency of chemical vulcanisation agents, especially sulphur and peroxides, during vulcanisation of the rubber compound. Poor crosslinking of the compound will result in a weak and undurable compound. To overcome this problem, electromagnetic radiation has been used to stimulate vulcanisation of the rubber compound, either solely, or in combination with chemical vulcanisation agents.
EP publication no. 0 204 531 A2 discloses a method of producing a rubber compound including the addition of chemical vulcanisation agents. The rubber component used is acrylonitrile-butadiene rubber and the chemical vulcanisation agent is peroxide. The use of chemical agents is undesirable due to its toxic nature and the potential damage to the environment. This publication teaches that the rubber compound is irradiated with low energy electron beams prior to carrying out the chemical vulcanisation process. Acceleration voltages used for irradiating the rubber compound ranges between 100 to 500 kV i.e. low energy electron beam. Low energy electron beam radiation was observed to penetrate the rubber surface only up to a few millimeters in depth. Hence, this method of vulcanisation is only effective for vulcanising very thin pieces of rubber e.g. up to 2 mm thick. No electrically conductive component is added to the rubber compound to prevent electrostatic build-up. Rather, the compound of this EP prior publication aims to provide good electrical insulation.
MY patent no. MY-121912-A discloses a method of producing a rubber compound with no added chemical vulcanisation agents. Vulcanisation is stimulated by exposing the rubber to a low energy electron beam while it is agitated within an irradiation vessel. The rubber component used includes natural rubber, synthetic rubber or a mixture of natural and synthetic rubbers such as acrylonitrile-butadiene copolymer latex. However, similar to the above EP publication, no electrically conductive components were added. From the teachings of the MY patent, it is noted that a low energy electron beam is used i.e. acceleration voltages of between 200-500 kV, and this resulted in penetration of only up to 1 mm in depth of the rubber compound, hence, the need to agitate the compound during vulcanisation. Such a method of agitating the rubber compound while radiating the compound with low energy electron beams resulted in a significantly prolonged vulcanisation time of up to 60 minutes.
US publication no. 2011/0049436 A1 discloses a rubber composition comprising acrylonitrile-butadiene rubber having carbon black as an electrically conductive component. This US publication teaches a vulcanisation process carried out using electron beam radiation with doses of 18 Mrad (180 kGy). It takes around 22 hours to complete the vulcanisation process i.e. low energy electron beam is used. As discussed above, the use of carbon black as an electrically conductive component is undesirable in many consumer applications due to staining.
Hence, there is a need for an electrostatic dissipative rubber compound for dissipating accumulated electrostatic charges without the undesirable properties such as staining and also without using chemical vulcanisation agents which are harmful to the environment.
It is also desirable to reduce the vulcanisation curing time in order to increase production levels by shortening the processing time.
This invention thus aims to alleviate some or all of the problems of the prior art.